When multiple communication nodes perform data communication with each other through a transmission line, with an increase of data rate, a transmission waveform is more likely to be distorted. At this time, the distortion of the transmission waveform results in an intersymbol interference, which interrupts digital communication processing from being properly carried out.
When the transmission path characteristic of the transmission line has been recognized in advance and a communication apparatus includes an emphasis circuit, it is possible to compensate for waveform distortion and properly perform data communication by minimizing the influence of the distortion. In general, emphasis circuit has a pre-emphasis type and a de-emphasis type, which have substantially the same implementation and substantially the same effect.
For example, the de-emphasis type is used in the internal bus of, e.g., a personal computer or the like. In the internal bus of the personal computer, consideration is given to waveform distortion which causes, e.g., a given transmitted bit to affect an immediately subsequent bit.
Generally speaking, when considering the influence of a given transmitted bit on an immediately subsequent bit, compensation may be carried out for compensating for the distortion caused by the influence. However, the transmission characteristic of a transmission line varies depending on a cable length, a material, or the like. Thus, it is difficult to identify the transmission characteristic when a situation cannot be recognized in advance.
As another technique which improves waveform distortion, there is a technique referred to as Adaptive DFE (Decision Feedback Equalizer). According to the DFE technique, a reception unit compensates for distortion using a reception waveform.
When the DFE technique described above is used, a circuit scale tends to increase. Suppose that a communication node which transmits normal data is a first communication node and a communication node which receives the normal data is a second communication node. For example, when the second communication node is more severely limited by size constraints than the first communication node, it is difficult to provide the DFE circuit in the second communication node. There may also be cases where, under various constraints which are not limited to size constraints, the second communication node cannot include a distortion compensation circuit such as the DFE circuit.
A technique in which the first communication node transmits a test pattern to the second communication node and a result of the test pattern reception is transmitted back to the first communication node is also known for compensating for the distortion (see, e.g., Patent Literature 1). However, this configuration requires an additional communication circuit in the second communication node for transmitting back the result of test pattern reception from the second communication node toward the first communication node.